Balanced electron tube circuits



Jan. 4, 1938.

W'.,SOLLER BALANCED ELECTRON TUBE CIRCUITS I 3 Sheets-Sheet 1v Filed Sept. 9, 1933 .95 /00 lai //0 lli /20l I., camu-m7' /fv MA.

Jan'. 4, 1938. i w SOLLER 2,104,211

BALANCED ELECTRON TUBE CIRCUITS Filed Sept. 9, 193s s sheets-Sheet s v Al /A/PUT Patented Jan. 4, 1938 "UNITED STATES 'PATENT OFFICE BALAN CED ELECTRON TUBE CIRCUITS Walter Seller, Tucson, Ariz., assignor of one-half I to William B. Woodin, Jr., Tucson, Ariz.

Application September 9, 1933, Serial No. 688,833 l? Claims. (Ci.- F75-183) My invention relates broadly to electron tube circuits and more particularly to the use of electronstube devices in measuring and amplifying electric voltages and currents.

One of the objects of my invention is to pro-I A further object of my invention is to provide a multi-stage electron tube ampliilcation system for signal receiving circuits having means for precisely balancing each of the electron tube stages.

A still further object of my invention is to provide a system ofbalancing a multi-stage ampliflcation system employing screen grid electron tubes, which involves the disposition of resistors of selected value between the output circuit of one electron tube and the inputcircuit of a succeecling'` electron tube in such arrangement as to' balance the operation of the circuits.

Another object loi my invention is'to provide a circuit arrangement for a multi-tube amplification system operative to amplify either direct or alternating current and in which special arrangements and values of resistors are employed for coupling the output system of one tube with the input system of a succeeding tube while insuring balanced operation thereof.

Still another object of my invention is to-pro vide a circuit arrangement for a screen grid multi-tube `amplifier system with a balanced bridgecircuit including resistors of selected values in selected arms'thereoi for effecting-'a balanced transfer of energy from the output of one screen grid tube to the input -of a succeeding .screen grid tube. v

A further object of my invention is to provide a circuit arrangement for a measuring instrument which includes an electron tube employing an additional grid element connected in balanced relation to an output meter for effecting the precision measurement of minute currents or voltages.

A lstill further object o f my invention is to provide 'an improved light sensitive cell control lcircuit having'means for freeing the light sensitive cell from variations in line voltages by precision balancing of the electron tube circuit.

Other and further objects of my invention re- I side in the electron tube circuit arrangements as set forth in the specification hereinafter follow--A ing, by reference to the accompanying\drawings in which:

Figure l is a diagram of the circuit arrangel ment of my invention as applied to a galvanome- 4 ter type of measuring apparatus;

Fig. 2 is a graph showing the anode current v ersus filament current characteristics of differ- I ent tubes tried in the measuring circuits of my invention;

Fig. 3 is a diagram of a circuit arrangement for an electrometer type of measuring system embodying my invention;

Fig. 4 is'a diagram of the balanced electron tube circuit lof my invention as applied to a measuring instrument adapted to be energized from a direct current power supply;

Fig. 5 is a circuit diagram of a balanced electron tube system embodying my invention and in which the voltage applied to the screen grid is equal to the voltage impressed upon the plate.

Fig. 6 is a circuit diagram of a balanced electron tube system embodying my Ainvention and in. .which the voltage applied to theI screen grid is equal to the voltage impressed upon the plate; Fig. '7 is a circuit diagram of a balanced electron tube system embodying iny invention and in which/the voltage applied to the'screen'grid is greater than the voltage impressed on the plate:

Fig. Bis a circuit diagram of a balanced electron tube system embodying my invention and in which the voltage applied to the screen gridisugreater than the voltage impressed on then plate;

Fig. y9 is a diagram of the circuit of my invention as applied to the amplification of alternat- Y ing currents or pulsating .direct currents and adapted to be energized from an alternating currenty power supply. lin* this circuit, as in they other ycircuits shown in Figs. 10 and 11, where alternating current is the power supply, heater type screen grid tubes are employed wherein the v cathodel is brought to operating temperature by my invention in conjunction with a light sensitive cell.

My invention comprises la single-tube balanced circuit which has all the advantages of the twotube balanced circuit without extra equipment and eliminates most of the tedious manipulations necessary in that circuit to obtain a balance. `I

have obtained accurate measurements by the use of the circuits of my invention inthe measurement of currents smaller than currents of the order of 10-15 amperes.

The two-tube balanced circuit is a Wheatstone bridge circuit in which the tubes form two of the v arms of the bridge. For all changes, except for the external voltage impressed'on one of the grids, the galvanometer must read zero for balanced conditions. The balance of a simple Wheatstone bridge does not depend on the current in the battery circuit. However, the variations in the battery circuit alter the filament current, grid bias, and plate voltage which in turn change the platefilament resistance of the tubes. But if identical tubes are used, the same voltages impressed on both tubes, and the same current passed through both filaments, then-the plate-illament resistances will change together, and by suitable adjustments of the other resistances of the circuit, the bridge can be made to remain balanced in spite of bat-` tery circuit fluctuations.

The circuit shown in Fig. 1 is a single electron tube balanced circuit employing an electron tube shown at i, having an anode at 2, a cathode at I, a screen grid at I, and a control grid at 5,r as elementary parts thereof. The anode circuit com- Y prises resistances numbered 6, 1, and 8 connected to the control grid and to the cathode through resistance shown at I6. 'I'he input is impressed across resistance shown at I1 and the output, in this case, is connected to the galvanometer.

This circuit may be considered as a Wheatstone bridge arrangement in which: resistance shown at I constitutes one leg; resistance numbered 8,

, in series with resistance 9 paralleled with the intate the use of equations.

ternal resistance of the tube between cathode and screen grid, a second leg; the internal resistance between cathode 3 and anode 2 of the tube, the y third leg; and resistance shown at 6, the fourth leg. Ihe former two legs carry the filament current Io, and the latter two legs carry the anode current I1. 'I'he input applied to the control grid and cathode serves to vary the current inthe anode-cathode leg of the bridge. I shall refer now to the letter designations in Fig. l to facili- The dry cell E causes the potential balance across the galvanometer to depend on In if everything in the circuit remains constant. But the plate current I1 fluctuates with the grid and plate potentials and the lament current. As these all uctuate with In, they also cause the potential balance across the galvanomter to depend-on In. Now as there are two influences on the balance depending on Io, they -can be made to neutralize each other for all values aioaaii depend upon the characteristics of the tube and circuit. The simplest way to determine this dependence is to regard the galvanometer, the cell E, and the resistances Ro and R1 as a Dowling zero shunt. When a current Io is passing through Ro, then the requirement for no current through the galvanometer is:

AIo in Io and a corresponding change A11 in I1, the following equation must hold:

Substituting for R1 from il),

n EAL E AAL-LAI., IVN!l 'Ihe above is the value of Ro for balanced conditions in terms of E, I1, Io and the slope of the I1 vs. Io characteristics. R1 is then determined by Equation (l).

If E-is constant. it is evident that the characteristics must be such that the denominator is a constant for all values of In in order that Ro can be a constant. By putting the denominator` in differential form, equating it to a constant and solving the diiierential equation, we obtain the characteristics of I1 vs. Io necessary to have Ro a constant as follows:

Equation (5) is the general equation of a straight line. Therefore, if the portion of the characteristics near the operating value of Io is straight, the circuit will balance and remain balanced.

It is interesting to determine how critical the value of Ro is. Consider Ra to be incorrect by an amount ARo. 'I'hen substituting in Equation (3) we have the incorrect value;

and from Equation (l) R1=(Ro'Io-E) /I1. The potential across the galvanometer when the current In has changed to Io-l-AIo will then be:

By substituting the values for Ro' and R1, we

obtain AE-:ARQIAIo-(Il/IQAII] Just how critical the value of Rn is, thus depends also upon the I1 vs. In characteristics. If

- the characteristics are such that then AE isaero irrespective of AR, hence any value of Re in this case will balance the circuit.

But th more A/A11 deviates from lo/I1, the more critical the value of Ro becomes. Equation (7) also shows that Re can be determined experimentally.. For Ra too large, AR@ is positive; hence, the galvanometer current will change in the positive direction with increase-of In. For Re too small, ARo is negative and, therefore, the galvanometer current will change in thenegative direction with increase of Io. By changing Io and noting the deiiection of the galvanometer, a value of Ro can be found for which changes in Io produce no change on the galvanometer.

The first experiment is the determination of the I1 vs. Io 'characteristics of several pliotrons connected as in Fig. 3. Fig. 2 shows the characteristics of four similar pliotrons with R:=13.6 ohms and 'R3-:18.1 ohms. The characteristics shown at I8 and I9 with theseresistances are very straight inthe operating region (110 lmilliamperes). They, therefore, flt the theoreticalA conditions for balancing. The characteristic shown at 2l is practically straight at 110 milliamperes; so it also ts the condition for balancing. The-characteristic shown at is' considerably more curvedthan the other three, and consequently. this tube should be more diiilcult to balance than the others.

The circuit of Fig. 3 is generally similar to The resistance across the input in the control grid circuit of Fig. 3. .Resistance at I6 in Fig. 1 is omitted in Fig. 3.

The correct value of Ro for any one of these tubes is determined experimentally by changing R. in Fig. l or Fig. 3 and noting the direction of deflection of the galvanometer, as described in the foregoing theory.' The degree to which the circuit is balanced is determined by noting the back and forth drift which a high sensitivity galvanometer will make, due to natural battery voltage fluctuations, or by varying Io and noting the galvanometer change. 1t was found that the tubes of I8 and I9 could be easily balanced and would remain balanced, and that the tube of 2| could be balanced and was stable. On the other hand, the tube of 2Il was difiicult both to balance and to keep balanced. -These results correspond with what was to be expected from the theory and the I1 vs. In characteristics of the tubes. It is probably possible to change the values of Rn andRz and obtain an Il vs. In characteristic for the tube of 2li which will be straight. at some workable value of Io. It should also be noted that the slope of the I8 and I9 curves approaches more nearly the conditions of Equation (8) thamthat of .2I, so

that "Ro is less critical for I8 l,and I9v than for 2|, which accounts for the experimental fact that the tubes 'of I8 and I9 are more easily balanced than the tube of 2l. l

The amount of variation in the circuit which can be balanced was determined for the tube of I8 as follows: The currentIo was changed 10% after the circuit had been balanced. This change produced only a few mm. deflection on the gallvanometer which was of a sensitivity about 10-s amper'es per mm. The large range in uctuation of the battery current which can bebalanced is ind icated by this experiment. I f

The choice of circuit Fig. 1 or Fig. 3 depends upon the magnitude of the current to be read and the time requirement for the reading. 'I'he pliotron and galvanometer arranged as in circuit Fig. 1 form a galvanometer type of system, which will read currents down to 5X 10-14 amperes.

The galvanometer can be made critically damped for resistance Ri and R0 by means of a damping frame. With this refinement and proper shieldup in a short time with ordinary laboratory rheostats, a set of lead cells E, not necessarily specially conditioned, `and a good dry cell. E. -v

The resistance R0, can be adjusted to its proper value forbalancing as closely as desired. The larger currents are very easily measured, and currents of 10-15 amperes can be measured with ljust ordinary precautions of insulation and shielding. In either this or the first circuit, E can be a standard cell, as only currents of galvanometerjmagnitude pass through it, and R1 can be adjusted so that no current is ever taken from the cell. For a permanent set-up, E is a standard cell, R0, R2, and R3 are permanent resistances,and the galvanometer is critically damped.

For the measurement of currents smaller than 10-15 amperes, it is desirable to neutralize the internal -grid current of the tube which is of this order of magnitude. To accomplish this, the small radioactive ionization cell, shown at 25, is placed in the circuit and the following adjustments are made: With the key at 24 closed', Ro is adjusted until the circuitis balanced close enough for currents of the magnitude which are to be measured. With no input current, key 24 is then opened and the cell 25 adjusted until the drift due to the gridv current is neutralized by the leak through the cell.

The grid current is very nearly constant with tron, and if the ionization cell 25 is at saturation voltage, the leak through cell 25 will not change with the grid voltage. The voltage on the grid, therefore, will be built up only by the input grid current.

After the circuit is adjusted, the rate of drift method, just as in the unneutralized'set-up, is applied to measure the very small currents.

vWith good shielding and the temperature of the tube constant, the circuit was found to be sumciently stable to measure currents of the order Ias an unbalanced'circuit, will operate much more satisfactorily, and with refinements, will balance as closely as the two-tube circuit.

The advantages over the two-tube circuit are: the elimination of the difliculty in balancing, of extra apparatus, and of the necessity of obtaining exactly similar tubes, which, from the characteristics shown here, are not readily procured.

, change of grid voltage at -4 volts for this plio- 50 indicated at 3|.

. aioaaii The balancing in these circuits is accomplished by adjusting resistances R0, R1, and voltage E in Fig. 3. Without voltage E, the circuit would not balance even though it appears the drop Ra I@ could be made to balance drop R1 I1. This is not possible, as can be seen, by allowing E to equal zero in the first equation after Equation (2) which will then cause Ro to cancel out of the equation and thereby eliminate the means of l0 adjusting for balance. To have an additional ,voltage E in the circuit R0, R1, and the output, 'is an essential feature of these circuits. Notice that the balance does not depend on the change of bias of the control grid, as there is no bias con- 16 trol in the balanced circuit of Fig. 3, and also that the screen grid was not used to balance the circuits shown where the additional cell E was inserted.

Fig. 4 shows the balanced single tube circuit of 20 my invention connected to a single D. C. supply, indicated at 21. The cell voltage shown at I2 in Fig. 3, is replaced by the voltage drop across the resistance shown at 28. The output meter at 29, may be the galvanometer shown at I0 oi' Fig. 3 or any device to be operated by the ampli' ed input current. The voltage on the screen grid in this circuit is less than the voltage on the anode by the drop across resistance shown at 30.

Fig. 5 is a diagrammatic sketch of the circuit of my inventionwherein the screen grid potential is equal to the anode potential.

Fig. 6 shows a variation of the circuit of my invention wherein batteries are replaced in the cirl cuit as shown at 38, 39 and 40. The screen grid potential is vequal to the anode potential and the respective currents are balanced.

Fig. '7 illustrates another variation of the circuit of my invention wherein the plat potential is less than the screen grid potential by the potential drop across resistance 4I.

Fig. 8 shows a variation of the circuit of Fig. 7 wherein batteries are replaced in the circuit, as shown at 40 and 42. The screen grid potential is greater than the anode potential by the voltage in the battery at 42 between the anode tap at 43 and the screen grid terminal at 45. I

Fig. 9 shows the circuit of Fig. 4 adapted for connection to an .alternating current power Source A second balanced circuit is connected to the output of the single stage shown in Fig. 4 for further amplication of the input.

Primed reference numbers in the second electron tube circuit denote elements similar to tlieparts 55 of the rst electron tube circuit which are similarly numbered. Alternating or pulsating direct current Vcan be amplied in this arrangement wherein inductive coupling is employed between the stages, as shown at 34, and a single source of anode potential, shown at 32, is connected to the anode of each tube. 'I'he screen grid voltage is less thanthe anode voltage by the drop across resistance shown at 30.

'as Fig. l0 shows the circuit of Fig. 9 but yemploying separate anode supply sources for each tube, as shown at 35 and 3B. 'I'his permits direct connection of the output of the first stage, to the input of the second stage, as indicated at 31, and

70 thereby the amplification of steady direct currents as well as alternating currents. Primed reference numbers in the second electron tube circuit denote elements similar to the parts of the ilrst electron tube circuit which are similarly u numbered. The voltage on the screen grid is less than the voltage on the anode by the drop across resistance shown at 30.

Fig. 1l is a diagrammatic sketch of the balanced ampliiier of my invention applied to a circuit containing a photo-electric device shown at 46, i

. in -which the phototube is not subjected to varia--` tions of line voltage.

The circuits of my invention show methods of taking the output of an electron tube circuit from between the anode and a grid, which is usel ful not only for balancing. and stabilizing the tube circuit, but as a form of amplier circuit to be used for any purpose where direct or alternating currents or voltages are to be amplified. The circuits of my invention comprise agnetwork 11 of resistances and the balancingY is accomplished by the network itself, permitting amplification of a given input independent of its frequency or phase.

The method of taking the output from anode 2' and a grid results in a higher degree of amplification than heretofore obtained besides balancing the circuit.

The voltage indicated'as |2'in the drawings and as E in the equations is an essential character- 21 istic of those circuits as described hereinbefore,

especially by reference to the equation following Equation (2). In circuits wherein this voltage is dispensed with, a voltage drop or other equivalent 3( means is substituted.

Wherever in the speciilcation and claims I have referred to th'e screen grid of the electron tube, I desire that it be understood thatany grid element additional to the control grid may be employed and is intended to be included by the said term screen grid. Either grid of a pentode may be employed in the circuit arrangement set forth herein. 'I'he circuits will operate with any tube employing a grid between the illament or 4( cathode and the control grid.

l The measurement of minute currents and voltages is but one application of the amplifying property of the stabilized circuitof my invention. Such a circuit may be used as a relay, or in line wire systems and circuits as a repeater or booster and in many other uses which require substantial and undistorted increase of the energy impressed on-the input of the amplifying device.

Although I have described my invention in certain of lts preferred embodiments, Idesire that it be understood that my invention is not to be limited thereby but may be modifled in arrangement and no limitations are intended other than are imposed by the scope of the appended claims.

, What I claim as new and desire to secure by Letters Patent of the United States is as foilows: Y Y

1. In an electron tube circuit, an electron tube device comprising an anode, a cathode, a control grid and a screen grid, an output circuit comprising a plurality of variable resistance elements connected in series between said anode and said cathode, an input circuit connected between said control grid and said cathode, indicating means, a source of constant voltage, said indicating means and said constant voltage source connected in series, said series combination connected in parallel with two of said variable resistance eiements connected in series, a second source of lwtential, one terminal of said second source of potential connected to the mid connection oi. said two variable resistance elements, the other terminai of said second source of potential connected to said cathode, means for.,extemaliy varying u y y 2,104,211 y the potential on Vsaid control grid through said input circuit, and means including said source of constant voltage for normally maintaining a balance of potential across said indicating means.

2. In an electron tube circuit,van electron tube device comprising an anode, acathode, a control grid, and a screen grid, a bridge arrangement comprising a plurality of variable resistance elements as two of the legs, the internal anode to cathode resistance as another leg, the internal cathode to screengrid resistance in conjunction with a plurality of resistance elements as the fourth leg, a source of potential connected through another variable resistance element to the cathode and the junction of the said rst two legs, indicating means connected to the junctions of the legs opposite junctions connected to said source of potential, means for producing a balance of potential across the terminals of said indicating means in said bridge arrangement and means for externally varying the current in one` of said legs, said external variation thereby aifecting said indicating means. A

3.- In an electron tube circuit, an electron tube device comprising an anode, a cathode, a control grid and a screen grid, a circuit comprising a plurality of variable resistance elements connectedv in series between said anode and said cathode, an indicating device 'and a sourceof potential diierence connected in series between said anode and a 4point in said series of resistance elements,

means for supplying potential for said anode andV said screen grid, an input circuit comprising a source of potential and a switch connected in series between said cathode and said control grid, said input circuitV also comprising, in parallel with said series, a radioactive ionization cell and a second source of potential connected in series,

said radioactive ionization cell acting to neutralize the internal control grid-cathode current of said electron tube device, and means for impressing minute electrical values -on said input circuit, the potentials on the terminals of said indicating device being normally balanced whereby said minute electrical-values may be measured by the degree of displacement of said balance.

4. In an electrical measuring instrument, an electron tube including a cathode, -a control grid, a screen grid and an anode, an input circuit connected with said control grid and cathode, an output circuit connected between said anode and said cathode, a measuring instrument connected in said output circuit, a resistor disposed in said output circuit in series with. said measuring instrument, a separate resistor connected between said screen grid and theterminal of said measuring instrument connected with said iirst mentioned resistor, and a potential supply circuit for said electron tube circuitextending from a source `olf potential to one side of said cathode, a pair of resistors, a. connection between the other side of said potential source with the junction of said pair of resistors, a connection between the end of one of said'resistors and said anode, and a connection between the end of the other of said resistors with the' resistor in the said output circuit at the terminal thereof connected with said 1 cathode. whereby a balance of potential across the terminals of said measuring instrument is normally maintained and minute electrical values,

impressed on said input circuit, are measured on said measuring instrument.

5. An electrical measuring'instrument including an electron tube having a cathode, a control grid, an anode and a screen grid, a measuring inconnected with strument, a source of potential, aseries circuit for supplying power from said source to said cathode, anode, and'screen grid, comprising a multiplicity of series'connected resistors, a connection from a point Ibetween two of said resistors to said screen grid, a resistor in said connection,

'a separate resistor .connected between a point in said series circuit and said anode, and connections between the opposite ends of said last mentioned resistors with said measuring instrument.

6. An electrical measuring instrument containy ing one or more electron tubes each having an anode, a cathode and at least one grid electrode, al1 electrodes of the same tube being supplied by one source of power, a series circuit including a plurality of impedance elements interconnecting said anode andv cathode electrodes, and means whereby the drop in potential across an impedance or impedances carrying the current of one electrode can`be balanced by thev algebraic sum of a drop of potential across an impedance carrying current directly from said source of power and either by the drop in potential across an impedance carrying the current of any other relectrode `of the same tube or by any separately introduced source of potential of any kind.

7. An electrical measuring instrument comprising an electron tube having anode, cathode, and a plurality of `grid electrodes, a source of power for said instrument, a balanced circuit connected withthe output of saidelectron tube, said balanced circuit including an impedance element carrying the current of said anodel electrode, a

,second impedance .element carrying current directly from said source of power, and a third impedance element carrying the current of one oi.' said grid electrodes, the algebraic sum of the potential drops across said impedance elements being zerofor a balanced condition of said circuit, means for impressing the quantity to be measured on another of said grid electrodes, an indicating device connected in said circuit and operable under conditions of unbalance of said circuit to indicate the degree of unbalance of said circuit and measure the value of the electrical quantity producing said unbalance.

8. An electrical measuring instrument as defined in claim I in which the means for impressing the quantity to be measured comprises a photo-electric device having an anode and a light sensitive cathode electrode, said anode being connected to the positive terminal of the source of power, and said light sensitive cathode being connected to the control grid for producing variations in the balance of the output circuit in accordance with light variations on said light sensitive cathode.- .v 9. An electrical measuring instrument comprising an electron tube having anode, cathode, and a plurality of grid electrodes, a source of power for said instrument, a balanced circuit the output of said electron tube, said balanced circuit including an impedance element carryingthe current of said anode electrode, a second impedance element carrying current directly from said source of power, and a separate source of potential, the algebraic sum of the potential drops across said impedance elements and thev separate source of potential being zero for a balanced condition of pressing the quantity to be measured on another of said grid electrodes, an indicating device conanother potential difference produced said circuit, means for imput of a single electron tube which comprises opposing a drop in potential due to the current flowy ing to one electrode of said tube by the algebraic sum of a drop in potential due to current flowing directly from a source of power and a separate potential diierence produced either as a drop in -potentialldueto the current owing to another electrode of said tube or in a separately introduced source of potential.

11. 'I'he method of balancing potentials across the terminals of an indicating device in the output of a single electron tube which comprises opposing a drop in potential due to the current flowing to one electrode of said tube by the algebraic sum of a drop in potential due to current flowing directly from a source of power and a separate potential diierence produced as a drop in potential due to the current owing to another electrode of said tube.

12; The method of balancing potentials across the terminals of an indicating device in the output of a single electron tube which comprises opposing a drop in potential due to the current ilowing to one electrode ci' said tube by the algebraic sum of a. drop in potential due to current flowing directly from a source oi.' power and a separate potential dierence produced in a separately introduced source of potential. Y

13. The combination of an electron discharge device having a cathode, a gridelement, a control grid element and an anode, a circuit including a source of voltage connected with said cathode, means for obtaining vfrom said circuit a voltage ior said anode, means for obtaining from said circuit a voltage for said rst grid element, an electromagnetic instrument connected between said anode and said rst grid element, and*V means conl nected between said latter means and said rst grid element forcompensating the changes in voltages supplied by said circuit.

14. The combination of an electron discharge device having a cathode, a grid element, a control grid element and an anode, a circuit including a source of voltage connected with said cathde, means for obtaining from said circuit a voltage for said anode, means for obtaining from said circuit a voltage for said ilrst grid element, an electromagnetic instrument connected between said anode and said rst grid element, and means connected between said latter means and said rst grid element for compensating the changes in voltages supplied by said circuit through a range of at least change.

15. In a measuring instrument, the combination of an electron discharge device having a cathode, an anode and two grid elements, an input circuit connected between said cathode and one of said grid elements, a circuit including a resistor connected between saidcathode and said anode, an indicating device and a resistor connected in series between said anode and an intermediate point on said iirst resistor, one terminal of said indicating device being connected to said anode, and a connection from said other grid element to a point between said indicating device and said second resistor.`

16. In a balanced circuit, the combination of an electron discharge device having a cathode, an anode, a control grid element and a grid element,

a resistor and a source oi' voltage connected ini series with said cathode, a resistor connected between said source of voltage and said anode, an electromagnetic instrument and a resistor connected between said anode and an intermediate point on said nrst resistor, and a connection from said rst grid element to said indicator and said last-mentioned resistor.

17. In a measuring circuitthe combination of an electron discharge device having a iilamentary` cathode, a space charge grid, and an anode, a resistor and a source of voltage connected in series with said cathode, a resistor connected between said anode and said source of voltage, an-

electromagnetic instrument connected between said anode and an intermediate point on said rst resistor, a connection from said space-charge grid to said intermediate'point on said tlrst resistor, and a resistor connected in series with said instrument and said connection i'or compensating' the changes in voltage of saidsouce o! voltage. WALTER somma. 

